The Astronomers' Universe Series is aimed at the same people as the Practical Astronomy Series - in general, active amateur astronomers. However, it is also appropriate to a wider audience of astronomically-informed readers.
Because optical astronomy is a science that is rather at the mercy of the weather, all amateur astronomers inevitably have periods when observing is impossible. At such times they tend to read books about astronomy and related subjects.
When researching this market, it is quite surprising to discover how few books there are that are of direct appeal to "armchair astronomers". There are many "popular science" books about matters cosmological, but because of their general audience these all start from the beginning, covering and re-covering the basics. At the other end of the spectrum there are professional books that are highly mathematical and technical, not intended for enjoyable reading.
The Astronomers' Universe Series begins by assuming an appropriate level of knowledge. Basic information about the distance, the solar system, galaxies, etc. is not part of these books, which can take a basic understanding of this as their starting point.
The series is differentiated from popular science series (such as Springer's Copernicus books) by a strong design image which will attract active amateur astronomers, and will also appeal to "armchair astronomers" (or cosmologists)and other readers who already have the necessary background knowledge.
The books have to be carefully written, structured and edited so as to be aimed at these scientifically-aware readers: they will have a background knowledge of astronomy and probably cosmology but many of them will not have formally studied science (amateur astronomers come from almost all walks of life) and will be discouraged by mathematical treatments. The content will therefore mostly be descriptive, with only essential mathematics included.